Scanning microscopy, such as scanning probe microscopy (SPM), e.g. atomic force microscopy (AFM), is known as an accurate and promising high resolution surface microscopy technique. This technology is for example applied in the semiconductor industry for mapping semiconductor topologies, particle and defect inspection and review and metrology. Other uses of this technology are found in biomedical industry, nanotechnology, and scientific applications. In particular, AFM may be used for critical dimension metrology (CD-metrology), particle and defect scanning, stress- and roughness measurements. AFM microscopy allows visualization of surfaces at very high accuracy, enabling visualization of surface elements at sub-nanometer resolution.
Scanning probe microscopy is usually performed by tracing of a sample surface in a scanning motion using a probe tip touching or tapping (i.e. repeatedly touching) the surface, while accurately measuring disposition of the probe tip in a direction transverse to the sample surface (z-direction) using for example a high precision optical sensing system, e.g. using beam deflection or an interferometer. Scanning is performed by vibrating the tip in the z-direction, while performing the scanning motion across the sample surface to be mapped. To map the sample surface, every fraction of a section of the sample surface with sub-nanometer dimensions is touched or tapped by the probe tip at least once, providing a highly accurate surface map.
Before scanning of the substrate surface may commence, the probe tip of the probe has to approach the sample surface sufficiently close to be able to perform the above tapping sequence. This may for example be achieved by an actuator system comprising a stepper motor. Such a stepper motor may typically have a dynamic range in the order of one or a few millimeters, and a sub-micron stepped increment resolution. The actuator system may move the probe accurately enough to approach the probe tip within operational distance for performing the scanning. However, the approach must be controlled such that the probe tip does not overshoot and crash into the surface, as this may break the probe and damage the surface.
In analogy, the challenge in controlling the approach method may be compared with controlling a mission to fly to the moon in 60 seconds and stop 38 meter from its surface without overshooting and crashing. Conventional methods often rely on repeated increments that are subsequently followed by operation of the scanner to sense whether the surface is near. This walk-and-talk method is comparable to moving blindfolded towards an obstacle by subsequently taking a pace followed by feeling with a hand whether the obstacle is nearby. As will be appreciated, this is a rather slow approach method that is not desired in, for example, an industrial environment wherein the throughput of surfaces to be mapped is relatively high.